A thermionic converter has been disclosed in, for example, JP 2004-349398 A. The thermionic converter converts thermal energy into electrical energy by using a phenomenon in which thermionic electrons are emitted from a surface of an electrode at high temperature. In JP 2004-349398 A, it is discussed that a distance between electrodes is reduced to the order of nanometers to improve thermionic electron emission efficiency by using the tunneling phenomenon in order to achieve high efficient conversion. Further, it is discussed that multiple thermionic converters are connected in series to obtain high electromotive force.
Regarding the first discussion, it is difficult to maintain such a small distance between the electrodes. Regarding the second discussion, the conversion efficiency may be reduced due to heat transmission from an emitter to a collector through a wire for connecting the thermionic converters together.
Further, when a temperature of the collector rises, back emission may occur. The back emission is a phenomenon in which thermionic electrons are emitted from the collector. Since the thermionic electrons emitted from the collector cancel the thermionic electrons emitted from the emitter, the conversion efficiency is reduced. It is necessary that a temperature of the emitter is higher than the temperature of the collector. In other words, a higher-temperature electrode acts as an emitter, and a lower-temperature electrode acts as a collector. Generally, the collector is cooled by a cooling device so that the temperature of the collector can remain lower than the temperature of the emitter.
The reference below has reported that when diamond semiconductor is used in an emitter and a collector of a thermionic converter, thermionic electrons are emitted from each electrode surface with very high efficiency because of the negative electron affinity (NEA) effect so that high efficiency conversion can be achieved at low temperature compared to metal.